1. Field of Invention
The present invention relates generally to bridge construction of the type using a concrete filled steel grid system for a deck and more particularly to bridge construction of the aforedescribed type utilizing a continuous base plate in the deck to which the grid system, floor system, and main supporting members are integrally connected.
2. Brief Description of the Prior Art
The concrete slab has been the most commonly used type of construction for highway bridge decks. Unfortunately, but realistically, this is not a completely satisfactory construction for a highway bridge deck. Concrete is a material well suited to carry loads in compression but, in addition to compressive loads, bridge decks are inevitably exposed to loads causing tensile stresses in certain areas on both the top and bottom of the concrete slab and concrete is not a very strong material under tensile stresses. Additional forces are applied to the concrete slab as a result of exposure to weather and large temperature variations which cause cyclic contraction and expansion. Also, concrete shrinks as it cures and ages. These conditions (temperature variations, shrinkage and loading) cause cracks in the top and bottom surfaces of concrete slabs. Continued exposure to temperature changes, and in most climates, numerous cycles of freezing and thawing over the years, cause further cracking and spalling of concrete bridge decks.
In some measure the cracking can be controlled, but not entirely eliminated, by steel reinforcement. The use of steel reinforcement embedded within a concrete slab presents a conflict to the bridge engineer. To be most effective in preventing cracking the reinforcement should be placed as near the surface of the concrete as possible. However, to prevent intrusion of moisture and corrosion of the steel, which can cause cracking in time, the steel reinforcement should be placed with as much concrete cover as possible. Corrosion of the steel reinforcement is one of the principal causes of deterioration of concrete bridge decks so prevalent in recent years with the increasing use of chlorides for de-icing. This is a major economic problem with highway bridge construction. An investigation in recent years reported that there were more than 50,000 bridge decks on the federal highway requiring major repairs or replacement, many in bridges with less than fifteen years service.
In recent years various procedures have been used to alleviate the problem of deck deterioration. These have included: (1) More concrete cover of the steel reinforcing; (2) protection of the steel by galvinizing or coating with epoxy; and (3) water-proofing the concrete slab. All of these procedures add very substantially to the direct cost of the bridge deck and indirectly to the cost of the remainder of the structure because of increased dead loads.
Concrete filled steel grid decks have given superior service and durability compared to the more common used concrete slab. An example of such a bridge deck is made by the American Bridge Division of United States Steel Corporation which for many years has made available a 41/4 inch or 3 inch concrete filled I-Beam-Lok bridge flooring. The flooring consists of a combination of special I-beams running longitudinally of the bridge, intersected at right angles with transverse cross bars. Metal form strips are placed between the I-beams and rest on the lower flanges of the I-beams. The transverse cross bars are securely interlocked with the main carrying I-beam. It is further specified that the entire unit be welded to the supporting members and concrete poured to a position flush with the top flange of the I-beams or to a three-quarter inch overfill above the top flange of the I-beams.
A shortcoming of the prior I-beam grid decks lies in the necessity to enhance the strength of the grid system in the lateral direction of the I-beams. This problem has previously been addressed by utilizing an upper and a lower set of cross bars so that continuous cross bar steel is maintained at both the top and bottom of the slab. Even with the additional set of cross bars, strength of the grid deck in a direction transverse to the I-beams is not as high for the weight of steel present as can be achieved.
A continuous base plate to which studs are welded and onto which concrete is poured has been utilized by a French bridge designer to form a bridge deck. Such a bridge deck has some of the advantages of the present invention, as for example in the use of a continuous base plate. This plate, however, is not stiffened by a grid system as in the present invention. Also, the concrete fill over the base plate has all the inherent shortcomings of the conventional concrete slab.